The present invention relates to an apparatus for generating an inductively coupled plasma under atmospheric pressure.
A conventional atmospheric inductively coupled plasma generating apparatus includes an impedance matching network circuit between a coil for generating an inductively coupled plasma under atmospheric pressure and an oscillator for generating a radio-frequency power (see, e.g., Patent Documents 1 and 2). The atmospheric inductively coupled plasma generating apparatus is hereinafter referred to as a plasma generating apparatus. The impedance matching network circuit includes a series capacitor and a shunt capacitor. At least one of the series capacitor and the shunt capacitor is mechanically controlled with a servo motor or the like, and undergoes a change in its capacitance. The series capacitor and the shunt capacitor are controlled so as to supply the RF power to the coil for plasma generation with maximal efficiency. Another conventional plasma generating apparatus includes a self-excited oscillation circuit including a coil for plasma generation as a part of a circuit, instead of including a mechanically controlled capacitor (see, e.g., Patent Document 3). The oscillation frequency of the oscillation circuit is controlled so as to supply a predetermined RF power to the coil for plasma generation. As another example, there is a plasma processing apparatus which generates an inductively coupled plasma under a pressure close to that in a vacuum (see, e.g., Patent Documents 4 to 10). Under a pressure close to that in a vacuum, plasma discharge is more easily initiated, and more easily maintained than under atmospheric pressure (according to the Paschen's law). In addition, because there are fewer gas molecules which could serve as a conductor under a pressure close to that in a vacuum, the apparent inductance of a coil for exciting an inductively coupled plasma is more stable than under atmospheric pressure.
Prior art documents we found are as follows: Japanese Utility Model Application Laid-open No. Showa 63-135799 (FIG. 1); JP Patent Laid Open No. Heisei 05-89824 A (FIG. 1); EP 0568920A1; U.S. Pat. Nos. 5,936,481A; 5,478,429A; JP Patent Laid-open No. 2000-49000 A; JP Patent Laid-open No. Heisei 09-161994A; U.S. Pat. Nos. 7,489,206B2; 5,688,357A; 7,306,745B1.
On the other hand, under atmospheric pressure, the impedance of the coil for plasma generation undergoes a significant change before and after plasma generation. In addition, the impedance of the coil for plasma generation may significantly fluctuate under the influence of an analysis target sample which is introduced after plasma generation. In the case of mechanically controlling the capacitance of the capacitor in the plasma generating apparatus, the speed at which the capacitance varies, that is, a matching speed presents a problem. For example, when the impedance of the coil for plasma generation has undergone a significant change, the plasma may become unstable or vanish before a proper matching state is restored. When the plasma has abruptly vanished, an extreme impedance mismatch may occur to excessively increase a load on an RF power source device, which may lead to a fault in the RF power source device.
In the case where the plasma generating apparatus has the self-excited oscillation circuit described above, there arises a problem that an amplifier element to be used cannot be freely selected. That is, because the amplifier element also serves as an oscillator element, a technology such as a power combining technology cannot be used, and hence it is necessary to select a high-cost element excellent in withstand voltage characteristics and loss resistance characteristics.
Further, because a plasma under a pressure close to that in a vacuum is different in characteristics from a plasma under atmospheric pressure, it is difficult to directly divert the technology used in the plasma processing apparatus to an atmospheric plasma generating apparatus.
It is therefore an object of the present invention to provide an atmospheric inductively coupled plasma generating apparatus including, as an RF power source for a coil for plasma generation, a separately excited oscillator, i.e., an oscillator which does not have the coil for plasma generation as a component of an oscillation circuit, in which impedance matching between the coil for plasma generation and the RF power source device is effected at a speed higher than in the case of using a matching network circuit based on mechanical control of the capacitance of a capacitor. To attain the above-mentioned object, according to the present invention, a control condition for the output frequency of the oscillator which supplies the power to the coil for plasma generation is appropriately changed in accordance with the generation state of the plasma. Alternatively, according to the present invention, control conditions for the output frequency of the oscillator and the output power thereof are appropriately changed in accordance with the generation state of the plasma. Further, according to the present invention, control methods therefor are appropriately changed in accordance with the generation state of the plasma.
Specifically, according to a first invention of the present invention, there is provided an atmospheric inductively coupled plasma generating apparatus comprising: an oscillator which is controllable in output frequency; a coil which is supplied with a power from the oscillator and generates an inductively coupled plasma with the power under atmospheric pressure; an impedance matching device disposed between the oscillator and the coil; a measuring device for measuring one of: respective phases of a voltage and a current in a line between the oscillator and the impedance matching device; a phase difference between the voltage and the current in the line; a parameter of a reflected wave in the line; and a ratio between the same parameters of a traveling wave and the reflected wave in the line; and a control device for controlling the output frequency of the oscillator based on a result of the measurement by the measuring device, the control device controlling the output frequency in accordance with a generation state of the atmospheric inductively coupled plasma.
Also, according to a second invention of the present invention, in the atmospheric inductively coupled plasma generating apparatus according to the first invention: when the inductively coupled plasma is not generated, the control device controls the output frequency according to a first condition; and when the inductively coupled plasma is generated, the control device controls the output frequency according to a second condition different from the first condition.
Further, according to a third invention of the present invention, in the atmospheric inductively coupled plasma generating apparatus according to the second invention: when the inductively coupled plasma is not generated, the control device controls the output frequency such that the phase of the voltage is caused to lead the phase of the current; and when the inductively coupled plasma is generated, the control device controls the output frequency such that the phase difference between the voltage and the current is reduced to zero.
Still further, according to a fourth invention of the present invention, in the atmospheric inductively coupled plasma generating apparatus according to the second invention: when the inductively coupled plasma is not generated, the control device controls the output frequency according to the first condition by a first control method; and when the inductively coupled plasma is generated, the control device controls the output frequency according to the second condition by a second control method different from the first control method.
Also, according to a fifth invention of the present invention, in the atmospheric inductively coupled plasma generating apparatus according to the fourth invention: each of the first control method and the second control method comprises PID control; and the second control method has a PID constant different from that of the first control method.
Further, according to a sixth invention of the present invention, in the atmospheric inductively coupled plasma generating apparatus according to the first invention: the oscillator is controllable in output power; and the control device controls the output power of the oscillator in accordance with a generation state of the inductively coupled plasma.
Further, according to a seventh invention of the present invention, in the atmospheric inductively coupled plasma generating apparatus according to the sixth invention: when the inductively coupled plasma is not generated, the control device controls the output power of the oscillator according to a third condition; and when the inductively coupled plasma is generated, the control device controls the output power according to a fourth condition different from the third condition.
Still further, according to an eighth invention of the present invention, in the atmospheric inductively coupled plasma generating apparatus according to the sixth invention: when the inductively coupled plasma is not generated, the control device controls the output power according to the third condition by a third control method; and when the inductively coupled plasma is generated, the control device controls the output power according to the fourth condition by a fourth control method different from the third control method.
Also, according to a ninth invention of the present invention, in the atmospheric inductively coupled plasma generating apparatus according to the eighth invention: each of the third control method and the fourth control method comprises PID control; and the fourth control method has a PID constant different from that of the third control method.
Further, according to a tenth invention of the present invention, in the atmospheric inductively coupled plasma generating apparatus according to the first invention, the oscillator is of a separately excited type.
According to the present invention, the impedance matching between the oscillator and the coil for plasma generation is effected by controlling the output frequency of the oscillator. Therefore, even when the apparent impedance of the coil for plasma generation has undergone a rapid change due to a sample introduced into a torch, it is possible to promptly cope with the change. As a result, unintended extinction of the plasma during the analysis of a sample such as an organic solvent is suppressed. In addition, the present invention individually provides control methods and control conditions for the output frequency and output power of the oscillator for each of the time of no plasma generation (i.e., the time when no plasma is generated) and the time of plasma generation (i.e., the time when a plasma is generated), and the methods and the conditions are selectively used, which renders the ignition of a plasma as well as the maintenance of the state where the plasma is generated easier than before.